1. Field of the Invention
The present invention relates to a toner for use in electrophotography, which is capable of producing high quality images with high resolution, and the method of producing such a toner.
2. Discussion of Background
In recent years, in line with the image formation by digital system of the electrophotographic process, a developer is required to reproduce high quality images. In addition, there are many chances of outputting digital images by use of a computer, digital camera, and scanner, and full-color copying machines and printers for producing hard copy of a full-color image have been widely utilized. Further, with respect to the computer for home or office use, many trials have been made to make such equipment compact, drop the manufacturing cost, and minimize waste products so as to increase the recyclability.
The toner for image formation in the electrophotographic process is conventionally prepared by pulverizing a mass comprising a resin and a colorant in a stream at high revolution, and separating the particles with desired particles size by classification. According to the above-mentioned conventional method, the particle diameter of the toner particles is controlled to be as small as possible, and the particle size distribution thereof is controlled to be as narrow as possible for improving the quality of the obtained toner image. However, the shape of the toner particles prepared by the above-mentioned pulverizing method is amorphous. Therefore, the toner particles thus obtained by the pulverizing method tend to be easily broken to pieces by the application of stress thereto. To be more specific, when such conventional toner particles are employed for a two-component developer, the toner particles are easily broken while stirring with a carrier in a developing unit. In the case of a one-component developer, the toner particles are also easily broken when coming in contact with a toner-layer-thickness regulator or a triboelectric charging blade. The thus generated finely-divided pieces of toner particles will lower the image quality of obtained toner images.
Further, the fluidity of the toner particles thus prepared by the pulverizing method is poor because they are amorphous in shape, so that a large quantity of fluidity-imparting agent is required. In addition, because of the amorphous shape, the packing of toner particles in a toner bottle is lowered, thereby preventing the equipment from being made compact.
For the formation of a full-color image, a full-color toner image formed on the surface of a photoconductor is transferred to an intermediate image transfer member and a sheet of paper. As mentioned above, the image transfer step becomes very complicated when a full-color toner is employed. In addition, the transfer performance of the toner particles prepared by the pulverizing method is poor. Therefore, the amount of toner required to form a toner image is necessarily increased, otherwise the toner image will not be transferred perfectly.
There is an increasing demand for the decrease of consumption of toner particles and the formation of high quality image with no partial omission, and further the reduction of running cost by increasing the image transfer efficiency. If the image transfer efficiency is remarkably high, it is not necessary to provide a cleaning unit for removing the toner particles remaining on the photoconductor or the intermediate image transfer member. This is capable of making the equipment compact, reducing the cost, and making use of the toner efficiently.
On the other hand, toner particles can be prepared by the suspension polymerization. To be more specific, an oily droplet comprising a monomer and a colorant is subjected to polymerization in water. By this method, the obtained toner particles are spherical. Therefore, the drawbacks caused by the amorphous shape of the toner particles obtained by the pulverizing method can be eliminated to some extent.
However, the suspension polymerization method has the shortcoming that it is difficult to prepare the toner particles in an intermediate shape between the spherical shape and the amorphous shape, which is considered to satisfy the image transfer performance and the cleaning properties at the same time.
Further, in order to increase the conversion from a monomer to a polymer in the course of suspension polymerization, it takes so much time to complete the polymerization.
In addition, when the wet polymerized particles are dried after separated from water, it is necessary to remove not only a water component but also a monomer component remaining in the porimerized particles. The removal of the monomer remaining in the polymerized particles is considerably difficult. This is because the toner particles obtained by polymerization tend to fuse and adhere to each other at a temperature lower than 100.degree. C., so that the temperature for drying the particles is limited. Thus, the toner particles are dried at low temperature under reduced pressure. However, it takes so much time to dry the particles, thereby increasing the manufacturing cost.
Further, if the toner particles are not completely dried in the course of the above-mentioned drying step, the toner particles tend to adhere to each other while stored at high temperature. This will cause the blocking phenomenon. Or the monomer remaining in the toner particles tend to ooze out to the surface of the particles during the storage at high temperature, so that the charging properties of toner are unfavorably changed. As a result, it is impossible to produce high quality toner images.
When the toner particles are prepared by the suspension polymerization, the area of interface is large, so that large quantities of dispersants such as a surfactant, inorganic finely-divided particles, and a water-soluble polymeric protective colloid are essential in the polymerization. The above-mentioned dispersants are apt to leave on the toner particles thus obtained by polymerization. Under the circumstances of high temperature, such remaining dispersant components will have an adverse effect on the triboelectric charging characteristics which are determined by the surface of the toner particles.
To solve the above-mentioned problem, it is conventionally proposed to sufficiently wash the obtained toner particles for the removal of the dispersant therefrom. However, in this case, a large amount of wash water is necessary, and the facility for drain necessarily increases the manufacturing cost of toner.
The suspension polymerization is regarded as one of bulk polymerization from a microscopic viewpoint. Therefore, controlling is difficult so as to obtain polymers having low molecular weights and classified in a narrow molecular weight distribution. This becomes a serious problem when a full-color toner is prepared by the above-mentioned suspension polymerization method. To be more specific, the quality of a full-color toner image is determined by the smoothness and transparency of the image. If the molecular weight of a resin component for use in the full-color toner is excessively high, the full-color image cannot be fixed with satisfactory smoothness and transparency by the application of the same energy as required to fix a resin with a low molecular weight. For example, a low-molecular weight polyester with excellent fixing properties cannot be obtained by condensation polymerization in water, so that such a resin cannot be applied to the above-mentioned conventional suspension polymerization method.
Furthermore, it is difficult to finely disperse a colorant such as a pigment in a monomer without a dispersant. Although the colored performance of the obtained toner particles is improved by using a dispersant, the dispersant has an adverse effect on the charging characteristics of the obtained toner. In addition, when the hydrophilic nature of the employed pigment is strong, the pigment tends to shift to the interfaces of the particles in the course of polymerization, so that unsatisfactory color development cannot be expected.
The fluidity of the polymer particles prepared by suspension polymerization is good because the prepared particles are spherical. However, when those polymer particles are used as the toner particles, the above-mentioned inherent fluidity is insufficient. Therefore, finely-divided particles of a fluidity-imparting agent are used in combination with the toner particles. In this case, however, there is the problem that all particles of the fluidity-imparting agent do not adhere to the surface of the toner particles, and the fluidity-imparting agent particles initially attached to the toner particles will easily fall off. Those finely-divided particles of the fluidity-imparting agent which are not attached to the surface of the toner particles will contaminate or damage the surface of the photoconductor. Further, a cleaning blade will be worn away by those finely-divided particles of the fluidity-imparting agent.
In the method of producing a toner as disclosed in Japanese Laid-Open Patent Application 7-181740, the amount of residual monomer component and organic solvent component is regulated in order to prevent a releasing agent from moving to the surface of the toner particles. Namely, when the volatile component such as a monomer and an organic solvent remains in the toner particles prepared by suspension polymerization, a resin for use in the toner particles tends to be plasticized, so that the releasing agent is apt to ooze out. In this application, however, there is no trial to reduce or completely remove such volatile components. Therefore, the preservation stability and the charging characteristics of the obtained toner particles are still insufficient for practical use.
By the way, the spherical toner particles can be prepared in an aqueous medium not by using the above-mentioned suspension polymerization method. For instance, a resin and a colorant are dissolved and dispersed in an organic solvent. The thus obtained mixture is emulsified to form droplets, and thereafter, the water and the organic solvent are removed from the mixture, thereby obtaining toner particles. This preparation method has the shortcoming that it is difficult to obtain the toner particles in an arbitrary intermediate shape between the spherical shape and the amorphous shape. In addition, this preparation method employs an organic solvent, so that there is the same problem caused by the residual monomer and solvent component as mentioned in the suspension polymerization method. In this case, the employed organic solvent remains before the drying step, so that the amount of residual organic solvent is considerably large. Therefore, in the course of drying step, the toner particles become so adhesive that the toner particles tend to easily aggregate to form large particles. Even if a low boiling point solvent is employed, it takes so much time to remove the solvent from the inside of the particles in the drying step. Insufficient drying has a bad influence on the preservation stability and the charging characteristics of the toner. Further, after the drying step, there are observed void portions in the toner particles, which are generated by the evaporation of the solvent. Due to such void portions, the obtained toner particles become fragile, so that they are easily broken. Image formation will be hindered by such broken pieces of the toner particles in the developing unit. Further, in the case of this method, since a large quantity of solvent is employed, it is required to add the step of collecting and recycling the solvent. This will cause the increase of manufacturing cost.
Furthermore, the above-mentioned emulsifying method also employs a dispersant to stabilize the droplets in water. Such a dispersant induces the same problem as in the above-mentioned suspension polymerization method. In order to minimize the amount of dispersant to be employed, it is proposed to use a self-emulsifiable resin. However, such a self-emulsifiable resin tends to be unevenly distributed on the surface of the toner particles, so that the charging characteristics of the obtained toner will be impaired.
Although the resin available for the above-mentioned emulsifying method is not so strictly restricted as the resin for use in the suspension polymerization, it is necessary that the resin be soluble in a non-aqueous organic solvent. Further, a colorant such as a pigment cannot be readily dispersed in a resin solution without any dispersant. The resin is not always stably adsorbed by the pigment in the solution. The dispersant and the fluidity-imparting agent will cause the same problems as mentioned in the suspension polymerization method.
According to Japanese Laid-Open Patent Application 7-325429, the toner is prepared by the above-mentioned emulsifying method, with the amount of residual organic solvent being controlled. However, to reduce the residual organic solvent to such an amount as specified in the above-mentioned application, it is necessary to carry out a deaeration treatment under reduced pressure over a long period of time. When the deaeration treatment is carried out at high temperature for the purpose of curtailing the drying time, the toner particles tend to aggregate in the drying step. Further, the problems of the generation of void portions in the toner particles, the restriction of kind of resin to be employed, and the use of a large quantity of solvent remain unsolved in the above-mentioned application. In addition, it is difficult to finely disperse the pigment when any dispersant is not employed.
Apart from the above-mentioned preparation methods, there is proposed a method of preparing toner particles in such a manner that silica is added to resin particles comprising epoxy resin, with the polarity of the silica being opposite to that of the resin particles, and the thus obtained mixture is heated in a liquid in which the above-mentioned resin particles are insoluble, thereby preparing spherical toner particles, as disclosed in Japanese Laid-Open Patent Application 3-217850.
In the above-mentioned preparation method, a large amount of silica is required in order to prevent the resin particles from being fused and attached to each other under the application of heat thereto. However, a mixture of the silica particles and the resin particles cannot be easily wetted by the above-mentioned liquid and dispersed therein. Consequently, those particles cannot be dispersed in the liquid, and coalescence of particles will take place. Further, it is necessary to remove the above-mentioned silica particles in a large quantity because the silica only serves as an agent for preventing the resin particles from being fused and attached to each other in the course of the preparation of the toner.
In addition, this preparation method cannot produce toner particles in an arbitrarily controlled form. Furthermore, it takes so much time to prepare the toner particles. Therefore, this method is unfavorable from the industrial viewpoint.